compiler.preprocess: add depends kwarg

This patch adds 'depends' keyword to compiler.preprocess().
It allows to execute other targets before doing the preprocessing.
Test-case is added to demonstrate that functionality: it
generates the header before preprocessing the C source that
uses that generated header.

Thanks to @bruchar1 for getting this patch to work.

1 files changed, 5 insertions, 0 deletions

diff --git a/docs/markdown/snippets/preprocess-depends.md b/docs/markdown/snippets/preprocess-depends.md
new file mode 100644
index 000000000..531246462
--- /dev/null
+++ b/docs/markdown/snippets/preprocess-depends.md
@@ -0,0 +1,5 @@
+## `depends` kwarg now supported by compiler.preprocess()
+
+It is now possible to specify the dependent targets with `depends:`
+for compiler.preprocess(). These targets should be built before the
+preprocessing starts.